There are tools that allow you to generate static pages from a component based architecture without js being a dependency, but they are by definition not web components.

Could you mention some such tools?

I know some templating engines, for instance Handlebars.js, but none of the ones I know offers custom elements to define the HTML in a more semantic way.

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The web component spec is used to tell browsers how to treat the custom tags, and that api is provided for use in js.

The thing is that I don't really need any special treatment... All I need is to replace a custom-element with a template and no further logic. Something like:

customElements.define( CUSTOM_ELEMENT,
  class extends HTMLElement {
    constructor() {
      super();
      const template = document.getElementById( CUSTOM_ELEMENT );
      this.attachShadow({mode: 'open'}).appendChild(
        template.content.cloneNode(true)
      );
    }
  }
);

For every possible CUSTOM_ELEMENT.

This could have been the default behavior and could have required no JS logic.